bnxt_en: split rx ring helpers out from ring helpers

To prepare for queue API implementation, split rx ring functions out
from ring helpers. These new helpers will be called from queue API
implementation.
Signed-off-by: David Wei <dw@davidwei.uk>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>